“Currency” is the Big Model’s name for the cause-and-effect relationships between factors of positioning and effectiveness, especially cue-mediated ones. (The Big Model doesn’t very effectively recognize pieces of game design that aren’t cue-mediated.)

Knights get +2 to hit.

Spend a hero point for a reroll.

If someone hits your character with a weapon, lose hit points equal to their weapon’s damage roll.

These are examples of what the Big Model calls “currency,” and what I’m calling “cue-mediated cause-and-effect relationships between factors of positioning and effectiveness,” and what a sensible gamer might call “mechanics.”